Enhancement of transverse properties of directionally solidified superalloys

ABSTRACT

THE MECHANICAL PROPERTIES OF THE COLUMNAR-GRAINED, NICKEL-BASE SUPERALLOYS IN THE TRANSVERSE DIRECTION ARE SIGNIFICANTLY IMPROVED BY THE ADDITION THERETO OF ABOUT 1 PERCENT ZICONIUM.

United States Patent 3,700,433 ENHANCEMENT 0F TRANSVERSE PROPERTIES OFDIRECTIONALLY SOLIDIFIED SUPERALLOYS David N. Duhl, Newington, Couu.,assignor to United Aircraft Corporation, East Hartford, Conn. NoDrawing. Filed July 12, 1971, Ser. No. 161,912 Int. Cl. C22c 19/00 US.Cl. 148-325 4 Claims ABSTRACT OF THE DISCLOSURE The mechanicalproperties of the columnar-grained, nickel-base superalloys in thetransverse direction are significantly improved by the addition theretoof about 1 percent zirconium.

BACKGROUND OF THE INVENTION The present invention relates in general tothe nickelbase superalloys particularly as unidirectionally solidifiedinto columnar-grained castings.

In the patent to VerSnyder 3,260,505, of common assignee with thepresent invention, reference is made to the production of castings, suchas turbine blades, by unidirectional solidification techniques. Castingsso pro duced have an elongated, columnar macro-grain structure withsubstantially unidirectional crystals aligned therein; that is, with acolumnar structure in the castings. As a turbine blade, the grainboundaries are oriented to be substantially parallel to the principalstress axis of the blades, and there is an almost complete eliminationof grain boundaries normal to this stress axis.

The mechanical properties of the columnar grain casting, including bothstrength and ductility, parallel to the columnar grains are superior tothose obtainable by conventional casting. The mechanical properties inthe transverse direction, however, viz perpendicular to the columnargrains, are not as high as those in the longitudinal direction. Withcurrent developments in cooled turbine hardware and the related thermalstresses incident thereto strength improvements in the transverseproperties of directionally-solidified castings are very desirable.

A particularly desirable alloy for turbine blade applications is thatreferred to in the VerSnyder patent as SM- 200 (now known as MAR-M200),having a nominal chemical composition of, by weight, 9 percent chromium,10 percent cobalt, 12.5 percent tungsten, 1 percent columbium, percentaluminum, 2 percent titanium, 1.5 percent iron, 0.015 percent boron,0.05 percent zirconium, 0.15 percent carbon, balance essentially nickel.In the patent to Gell et al. 3,567,526, the substantial elimination ofcarbon is recommended.

As hereinafter discussed in greater detail, the present inventioncontemplates the inclusion of about 1 percent zirconium in nickel-basesuperalloy castings characterized by a columnar grained microstructure.It is known in the art that the hot workability of alloys may sometimesbe improved by the addition of zirconium, cerium, hafnium or yttrium, asindicated in an article entitled A New Approach to the Problem of theWorkability of Nickel- Chromium Alloys, Parry et al., Journal of theInstitute of Metals (1969), vol. 97. Furthermore, certain prior artcompositions, such as those referred to in the patent to Freche et al.,3,276,866, frequently describe zirconium contents of up to two percentwhen referring to alloys "ice having sufiicient ductility to facilitatefabrication of the alloy into sheet. In describing the alloy, designatedWAZ- 20, the above inventors in the report NASA TN D-5352 available fromthe Clearinghouse for Federal Scientific and Technical Informationobserve that zirconium and trace amounts of other elements were pickedup from the crucible during induction melting.

SUMMARY OF THE INVENTION The present invention contemplates theinclusion of about 1 weight percent zirconium to the nickel-basesuperalloy castings characterized by a columnar grained microstructureresultant from controlled unidirectional solidification techniques, toprovide improved mechanical properties transverse to the direction ofcolumnar grain alignment.

A maximum zirconium content of about 1.25 weight percent is requiredwith the advantageous results being obtained within the zirconiumcontent range of about 0.75-1.25 percent.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The nickel-base superalloys arerecognized as those having high strengths at high temperatures such asthose associated with the hot section of gas turbine engines. It is fromthis class of alloys that blades and vanes for use in gas turbineengines are preferably formed. As a class, these alloys are usuallycharacterized by the inclusion of aluminum and/or titanium to promotethe formation of the Ni (Ti,Al) intermetallic (7' phase), strengtheninga nickel-chromium solid solution matrix. These alloys also normallycontain elements like cobalt, and the refractory metals such astungsten, molybdenum, etc. Typical of such superalloys are thosereferred to in the VerSnyder patent.

The addiiton of zirconium to the directionally solidified alloy castingwill significantly improve its properties in a direction transverse tothe growth direction. Illustrative of this effect is the enhancement oftransverse properties to the MAR-M200 alloy resultant from the additionof 1 percent zirconium as shown in Table I. At 1400" F. (ductilityminimum in this alloy) the increase in creep is most striking while at1800 F. the creep elongation is enhanced. The significant increase in1400 F. properties is most desirable in gas turbine hardware. It willalso be noted that at a zirconium content of 1.3 weight percent, thetransverse properties are inferior even to the basic MAR-M200composition which for the purposes of the TABLE I Weight percentZirconium (PWA 664) 0.8 1.15 1.3

Test F/K s.i. Life Percent E Life Percent E Life Percent E Lite PercentE A. Transverse:

1,400/100 i 1 2.8 224.5 5. 7 B.O.L. 1,800/25. 49 1. 3 2. 3 1,800] 40 1.35. 1 B. Longitudinal:

l B.O.L.=broke on loading.

With the zirconium addition, the transverse properties of thedirectionally-solidified columnar-grained castings have been brought tostrength levels in the transverse plane approaching the strengths in thelongitudinal direction, thereby reducing the extent of anisotropy inductility and strength therein. Furthermore, consonant with thetransverse property improvement, the desirable longitudinal proper-tiesof the columnar-grained material (enhanced thermal fatigue resistance,enhanced longitudinal creep properties and decreased longitudinalmodulus) are still retained.

The invention in its broader aspects is not limited to the specificsteps, process and compositions shown and described but departures maybe made therefrom within the scope of the appended claims withoutdeparture from the principles of the invention and without sacrificingits chief advantages.

What is claimed is:

1. In the production of a columnar-grained cast article by theunidirectional solidification of a nickel-base superalloy, the method ofimproving the mechanical properties transverse to the alignment of thecolumnar grains which comprises incorporating 0.75-1.25 weight percentzirconium in the superalloy composition.

2. As an article of manufacture, a cast article comprisa nickel-basesuperalloy containing about 0.75-1.25

weight percent zirconium characterized by a columnar grainedmicrostructure with the columnar grains substantially aligned parallelto the principal stress axis of the article and characterized further bystrength and creep properties transverse to said stress axis approachingthose parallel thereto.

3. As an article of manufacture, a cast article comprising:

a superalloy which consists essentially of, by weight, about 9 percentchromium, 10 percent cobalt, 12.5 percent tungsten, 1 percent columbium,2 percent titanium, 5 percent aluminum, 1 percent zirconium, up to about0.20 percent carbon, 0.02 percent boron, balance substantially nickel,characterized by a columnar grained microstructure with the columnargrains substantially aligned parallel to the principal stress axis ofthe article and characterized further by strength and creep propertiestransverse to said stress axis approaching those parallel thereto.

4. A cast article according to claim 3 wherein:

the cast article is a gas contacting blade.

References Cited UNITED STATES PATENTS 3,260,505 7/1966 VerSnyder -171RICHARD O. DEAN, Primary Examiner US. Cl. X.R.

